The easiest way to get the diameter of the axle is to put a wrench on it. It's probably 3/8 of an inch. Measure 1 inch of the length of the axle and count the high or low points of the threads. It's probably 26. This is a standard British thread size, but not interchangeable with American or Metric. I would think that CEI (Cycle Engineers Institute) wing nut will be hard to find. eBay has British taps available. Be aware that British Standard Fine (BSF) is not 26 threads per inch. Count your threads.